(1) Field of the Invention
The present invention relates to light-emitting display devices which display images and manufacturing methods for the light-emitting display devices, and particularly to a light-emitting display device including thin film transistors for driving an organic EL display and organic EL elements and a manufacturing method for the light-emitting display device.
(2) Description of the Related Art
In recent years, organic EL display devices using electro luminescence (EL) of organic material has been attracting attention as one of the next-generation flat panel displays replacing the liquid crystal displays. Unlike the voltage-driven liquid crystal displays, the organic EL display devices are current-driven devices, and are divided into the active-matrix type and the passive-matrix type.
For example, in an active-matrix organic EL display device, an image is displayed by current flowing in an organic EL element provided in each pixel, causing the organic EL element to emit light. The amount of current flowing in the organic EL element is controlled by a thin film transistor (TFT).
As a channel layer of such a TFT, an amorphous silicon film has been used, for example. However, there has been research and development for forming (micro) crystals in the amorphous silicon film to form a crystalline silicon film so as to increase the driving capacity of TFT.
In a pixel circuit in the organic EL display device, the TFTs are used as a driving transistor for supplying driving current to the organic EL element, and a switching transistor for supplying data signals according to the intensity of the driving current to a gate electrode of a current driving transistor. A significantly low off-current is required for the switching transistor, and a current (mobility) for driving the organic EL element, that is, on-current is required for the driving transistor. However, forming micro crystals in the channel layer portion of the TFT so as to increase the driving current (mobility) of the TFT increases the off-current of the TFT.
In response to this problem, a technology in which the TFT composing the switching transistor uses amorphous semiconductor for the channel layer, and the TFT composing the driving transistor uses a crystalline silicon film for the channel layer has been disclosed (for example, see Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2007-219517).